The present invention relates generally to transmissions for use in motor vehicles and, more particularly, to a twin-clutch automated transmission applicable for use in four-wheel drive vehicles.
Automobile manufacturers continuously strive to improve fuel efficiency. This effort to improve fuel efficiency, however, is typically offset by the need to provide enhanced comfort and convenience to the vehicle operator. For example, it is well known that manual transmissions are more fuel efficient than automatic transmissions, yet a majority of all passenger vehicles are equipped with automatic transmissions due to the increased convenience they provide.
More recently, xe2x80x9cautomatedxe2x80x9d variants of conventional manual transmissions have been developed which shift automatically without any input from the vehicle operator. Such automated transmissions typically include a plurality of power-operated actuators that are controlled by a transmission controller to shift traditional synchronized dog clutches. However, such automated transmissions have the disadvantage that there is a power interruption in the drive connection between the input shaft and the output shaft during sequential gear shifting. Power interrupted shifting results in a harsh shift feel which is generally considered to be unacceptable when compared to smooth shift feel associated with most automatic transmissions. To overcome this problem, automated twin-clutch transmissions have been developed which can be powershifted to permit gearshifts to be made under load. Examples of such automated manual transmissions are shown in U.S. Pat. Nos. 5,966,989 and 5,890,392. While such powershift twin-clutch transmissions overcome several drawbacks associated with conventional single-clutch automated transmissions, a need exists to develop simpler and more robust transmissions which advance the automotive transmission technology.
In four-wheel drive vehicles, it is common to use a transfer case for transmitting drive torque from the transmission to the front and rear drivelines. However, due to the size and weight of transfer cases, special accommodations must be made to package the transmission and transfer case under the vehicle. Thus, efforts have been made to integrate the components of the transfer case into the manual transmission. Examples of such integrated transmissions are shown in commonly-owned U.S. Pat. Nos. 5,704,866 and 5,904,632. While such integrated transmissions advance the four-wheel drive technology, a need exists to develop four-wheel drive transmissions integrating twin-clutch automated shifting systems and torque splitting systems into a common assembly.
Automobile manufacturers are also actively working to develop alternative powertrain systems in an effort to reduce the level of pollutants exhausted into the air by conventional powertrains equipped with internal combustion engines. Significant development has been directed to electric vehicles and fuel-cell vehicles. Unfortunately, these alternative powertrain systems suffer from several disadvantages and, for all practical purposes, are still under development. However, several different hybrid electric vehicles (HEV) have recently been offered for sale. These hybrid vehicles are equipped with an internal combustion engine and an electric motor that can be operated independently or in combination to drive the vehicle.
There are two types of hybrid vehicles, namely, series hybrid and parallel hybrid. In a series hybrid vehicle, power is delivered to the wheels by the electric motor which draws electrical energy from the battery. The engine is used in series hybrid vehicles to drive a generator which supplies power directly to the electric motor or charges the battery when the state of charge falls below a predetermined value. In parallel hybrid vehicles, the electric motor and the engine can be operated independently or in combination pursuant to the running conditions of the vehicle. Typically, the control strategy for such parallel hybrid vehicles utilizes a low-load mode where only the electric motor is used to drive the vehicle, a high-load mode where only the engine is used to drive the vehicle, and an intermediate assist mode where the engine and electric motor are both used to drive the vehicle. Regardless of the type of hybrid drive system used, hybrid vehicles are highly modified versions of conventional vehicles that are expensive due to the componentry, required control systems, and specialized packaging requirements.
Such hybrid vehicles have also been adapted to four-wheel drive vehicles which typically utilize the parallel hybrid powertrain to drive the primary wheels and a second electric motor to drive the secondary wheels. Obviously, such a four-wheel drive system is not only extremely expensive and difficult to package, but is also difficult to control in view of the need to react to instantaneous instances of wheel slip. Thus, a need exists to develop hybrid powertrains for use in four-wheel drive vehicles that utilize many conventional powertain components so as to minimize specialized packaging and reduce cost.
Accordingly, it is an object of the present invention to provide a four-wheel drive twin-clutch transmission and a control system for permitting automatic shifting of the twin-clutch transmission.
As a related object, the four-wheel drive twin-clutch automated transmission of the present invention has a compact geartrain and an interaxle differential for use as an integrated transmission/transfer case in a four-wheel drive vehicle.
A further object is to provide a hybrid powertrain or drive system for a four-wheel drive vehicle equipped with the twin-clutch automated transmission of the present invention.
These and other objects of the present invention are met by providing an automated twin-clutch multi-speed transmission adapted to transfer power from the engine to front and rear drivelines of a motor vehicle. The transmission includes a first engine clutch operable to establish a releasable drive connection between the engine and a first input shaft, a first motor/generator connected to the first input shaft, a second engine clutch operable to establish a releasable drive connection between the engine and a second input shaft, a second motor/generator connected to the second input shaft, a mainshaft, and a geartrain for selectively establishing a plurality of forward and reverse speed ratio drive connections between the input shafts and the mainshaft. The mainshaft drives an input to an interaxle differential having a first output adapted to drive the rear driveline and a second output adapted to drive the front driveline such that the transmission provides a full-time four-wheel drive mode of operation. The transmission further includes power-operated dog clutches for selectively engaging constant-mesh gearsets associated with the geartrain, and a transmission controller for controlling coordinated actuation of the first and second engine clutches, the first and second motor/generators, and the power-operated dog clutches to permit non-power interrupted (xe2x80x9cpowershiftxe2x80x9d) sequential gear changes automatically without input from the vehicle operator. When shifting under power between gear ratios, one engine clutch is released and the corresponding motor/generator is actuated to accelerate/decelerate the released input shaft into speed synchronization with the mainshaft. Following completion of speed synchronization, the dog clutch for the selected gearset on the released input shaft is actuated and thereafter the released engine clutch is re-engaged.
In accordance with a further feature of the present invention, the automated twin-clutch transmission can be equipped with a biasing clutch for automatically limiting slip and biasing the torque distribution between the front and rear drivelines for establishing an all-wheel drive mode of operation. In accordance with yet a further derivation of the automated twin-clutch transmission, the interaxle differential can be eliminated and a transfer clutch disposed between a primary (i.e., driven) output and a secondary (i.e., non-driven) output to establish an on-demand four-wheel drive mode of operation.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the scope of this invention will become apparent to those skilled in the art.